Briggs 23D swap

The changes I know of are mainly in the ignition system. The earliest 23 engines have an ignition system known as Magna-Matic. It's not the same as Magnatron since it still uses points where Magnatron is breakerless.  Magna-Matic has a timing advance mechanism. These engines will have the coil under the flywheel. Later model engines use the typical external coil next to the rim of the flywheel. There are other possible differences such as ball bearing mounted crank vs a plain bushing and a Stellite exhaust valve. All things considered, the early 23 would be better in my opinion.